Johnson Window Films Dealer Resource Center

JWF, Basic Process

Glass Breakage

Glass is susceptible to various types of breakage. The most common cause is due to impact, either accidental or intentional. However, there is another less obvious cause known as thermal stress breakage. There are several factors that produce thermal stress on a window and it is for the professional installer to recognize them when recommending a suitable window film.

Origin of Thermal Stress
Thermal stress fractures causes an uneven corner viewdistribution of heat across the surface of a window. Although stress fractures may happen anywhere on the glass, their origin is usually traced to a defect in the glass. The glass may have an obvious physical defect (i.e. chip, nick or depression) or an unseen source such as an uneven or damaged edge which is hidden by the frame and glazing materials.

As a glazing system is exposed to sunlight, it conducts heat by absorption. This heat causes the glass to expand within the frame. If the heat is unable to uniformly disperse itself throughout the pane, a portion of the glass seeks to expand quicker than the rest, which creates surface pressure. This sudden pressure potentially causes the glass to crack. While solar absorption in some instances may be a desirable effect, it is a critical factor to evaluate when inspecting a glazing system. A generally accepted standard is the 50% rule. A glazing system should not exceed an absorption rate greater than fifty percent. Slight variances to this standard measure but be cautious when exceeding this benchmark.

Causes of Thermal Stress
Now that you know the primary cause of thermal stress, it is Found the rough spotimportant to know other influences that trigger its effects. Anything that blocks sunlight from uniformly lighting a glazing system is a problem. Some obvious examples are trees, overhangs and awnings. Any variables partially shading a window should be noted, and a determination should be made as to the degree of affect that it has and decisions made concerning its effect on the glazing system. Interior window treatments may have an effect on a windows ability to convect heat. Shades, venetian blinds, shutters and drapes all trap heat, thereby increasing surface temperature.

The location of a window is also a factor when assessing breakage potential. Glass with a southerly exposure tends to be at greatest risk because it receives the greatest amount of sunlight (East and West facing lights are also at risk, but to a lesser degree). In colder climates, glass facing east is at risk because it receives early morning sunlight. During the winter months, when the glass has several hours to reach a low uniform temperature and then is subjected to early morning light quickly raising the temperature. West facing windows are subject to thermal stress during warmer afternoons and evenings, although their degree of vulnerability is less than south and east facing windows.

In the Southern Hemisphere, the opposite applies, it is the northerly exposure that tends to be at greatest risk of thermal stress.

What is a glazing system?Evaluating Glazing Systems to Avoid Glass Breakage
The next stage is to determine the type of glass. Standard annealed glass will have the greatest risk of breakage due to its minimal surface compression. Conversely, heat strengthened or tempered glass receives less thermal stress because of their more durable physical characteristics which tolerates uneven heat distribution.
Next, check glass thickness and size. In general, solar control film should not be applied to clear annealed glass where the square footage is greater than 100 s/f, and/or the thickness exceeds 3/8 inch. When dealing with tinted annealed glass, do not apply film where the width exceeds 1/4 inch. Solar control film is not recommended for clear or tinted laminated glass of any thickness or dimension.

There are several varieties of glazing materials on the market today. The type depends on when the windows were manufactured. Glazing materials create a seal between the glass and the frame and cushions the glass while it rests in the frame. Modern glazing materials are made from rubber, silicone, neoprene, foam tapes and other flexible sealants. Older structures may have a hard, clay-like putty as their primary means of adhesion. Regardless of the glazing material, inspect for evenness along the edges and a uniform texture throughout. A damaged or altered seal contributes to inconsistent thermal expansion.

Glass color also influences solar absorption. Factory tinted and coated glasses have higher rates of absorption than their clear counterparts. If possible, try to attain the window manufacturers specifications in order to accurately determine the amount of solar absorption.

The framing system of a window should also be evaluated. The framing system works in conjunction with glazing materials to cushion the glass, and keep moisture and air from entering the building. A damaged framing system may attribute to further unseen damage to the glass.

What is a glazing system?Why should you evaluate?
Adhering to the above guidelines promotes proper selection of the film for the job, and minimizes the risk of glass breakage caused by thermal stress. It's important to note that window film alone is rarely responsible for glass failure. However, film may amplify an existing flaw in the glazing system to the point at which the system fails. By performing a professional inspection and communicating your findings with a potential client, you minimize risks and increase your odds of being chosen for the project.

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